The application relates generally to the field of wireless digital data communications. The application relates more specifically to the detection and analysis of interference and noise within the bandwidth of a predetermined communication channel.
In wireless digital data communications, in particular satellite communications, policing of data communications channels is performed for Quality of Service assurance and fault diagnosis purposes. For example, in-band interference can seriously degrade the demodulated Bit Error Ratio (BER) of a receiver. A stable, high-quality communication route avoids data loss. If a chosen communication path is error-prone, it is likely that the transmitted data may get corrupted or even lost. Generally, the lowest BER provides the highest level of Quality of Service (QoS).
In satellite digital communications, such in-band interference may arise for various reasons. A user may unintentionally interfere due to system configuration error or due to a satellite transponder frequency planning problem. In other cases, in-band interference may be due to malicious interference by another party, also referred to as signal jamming. Interference may also be self generated due to the presence of a non-linearity in the communications path. In other cases, poor antenna or Orthogonal Mode Transducer (OMT) alignment may result in insufficient cross-polarization rejection.
Sometimes, interference in a received signal can be obvious when viewed on a spectrum analyzer, especially if the interference is narrowband and has a relatively high power compared to the signal of interest. However, if the interference is broader band and has a low power level relative to the signal of interest, then the interference may not be readily perceptible on a spectrum analyzer. The disclosed systems and methods address the need for the latter case whereby low level in-band interference requires detection and analysis within a data communications modem receiver.
Digital systems transmit information as either 0s or 1s. Accurate information transmission depends upon being able to reliably detect transmitted 1s and 0s. Accurate detection is not always a simple matter because of factors such as low received signal power and multiple transmission paths caused by reflecting objects between the transmitter and receiver. Error correction techniques have been developed to increase the accuracy of digital communications systems. A digital information sequence which has been subject to error correction coding must be decoded at the receiver. An optimum maximum likelihood decoder, for this type of coding, determines a sequence of bits which has a maximum likelihood of being the sequence that was sent. The Viterbi algorithm is a maximum likelihood decoding scheme for use at a receiver where an information sequence has employed an encoder using convolution codes and the channel is an additive white Gaussian noise channel.
In general, channel decoding can be performed in two ways, hard-decision decoding or soft-decision decoding in a soft symbol, domain. Usually, samples of the demodulated signal are quantized resulting in bits so that, at the output of a demodulator, decoding can be performed in a bit-wise manner. In the hard bit domain, the demodulator quantizes each sample to one of two levels, i.e. 0 or 1, and is said to have made a hard-decision. The decoder that works with this kind of input is said to perform hard-decision decoding. On the other hand, if quantization is performed using more than two levels per bit, the resulting quantized samples are called soft symbols, or symbols. A soft symbol is the amplitude of the I and Q channel baseband outputs of the demodulator sampled at each symbol period. For example, when using Quadrature Phase Shift Keyed (QPSK) modulation, the demodulated soft symbols would ideally lie on or, in the presence of noise, reasonably close to, four points on the complex plane arranged in a square constellation. The decoder making use of the information in soft-symbols is performing soft-decision decoding. For example, U.S. Pat. No. 5,802,116 issued Sep. 1, 1998 to Baker et al. presents a method and apparatus for obtaining a soft symbol decoded output of a received signal by a two pass Viterbi operation. U.S. Pat. No. 6,760,438 issued Jul. 6, 2004 to Hui, et al. discloses a system and method for Viterbi decoding on encrypted data.